FIELD OF THE INVENTION
The invention relates to a planar filter, in particular for multiple-pole plug connectors (plugs or counterpart plugs), being disposed in a housing that makes a ground connection, for signal lines carrying electronic signals, including a number of closely spaced capacitors corresponding to the number of signal lines and being disposed on a substrate, the capacitors being formed by one common ground electrode being essentially continuous at least in a peripheral region of the substrate, by individual signal electrodes being provided for each signal line and being insulated from the others, and by a dielectric layer between them, wherein the substrate has one opening for each of the signal lines, to or into which opening the signal electrodes extend and are connected there to the associated signal lines.
In the prior art, filters are provided in order to connect lines with which computers, for instance, or other electronic devices are connected for the purposes of signal exchange. The filters suppress electrical interference pulses that are the consequence of electromagnetic or electrostatic interference resulting from lightning or other events, for instance. Such filters are constructed as planar filters, for instance as described in French Patent No. 2 422 268, U.S. Pat. Nos. 4,791,391 or 4,070,084. In each signal connection, a capacitor or a capacitor and inductive resistor combination in the form of a Pi and L filter is provided as a filter element. The capacitors, which are generally made by thick-film technology, have limited electric strength, which is conditional on the material. The construction of the filter and the materials used for it are decisive for the magnitude of the electric strength. If barium titanate in the form of a thin strip or a screen-printed coating applied in paste form is used as the dielectric in the capacitors, for instance, then both the strip and the screen-printed application are fired after the joining or multilayer printing has been carried out. The resultant thin dielectric layers have a limited electric strength. Both the type of dielectric and its contamination from inclusions as well as the development of spines or sharp points on the coatings that as a rule are applied by screenprinting to the capacitors, play a role, since corona discharges can develop that cause channeling and are thus the source of dielectric breakdowns in the final analysis.
The electric strength of such planar filters is increasingly important for use in telecommunications, for instance. The present requirements in terms of surge voltages (test values of 1,000 V with a 10 .mu.s rise and a 350 .mu.s drop, for instance) in telecommunications, for instance, has exceeded the electric strength of many capacitors used until now for the desired filter properties (capacity, frequency). In order to increase the electric strength by increasing the layer thickness, a reduction in capacity must be accepted, although it could be compensated for by raising the dielectric constant, so the material constant might be "outbid". It therefore appears advantageous to improve the filter element so that it meets the higher requirements.